It is well known to use extruded tubes of polytetrafluoroethylene (PTFE) as implantable intraluminal prostheses, particularly vascular grafts. PTFE is particularly suitable as an implantable prosthesis as it exhibits superior biocompatibility. PTFE tubes may be used as vascular grafts in the replacement or repair of a blood vessel as PTFE exhibits low thrombogenicity. In vascular applications, the grafts are manufactured from expanded polytetrafluoroethylene (ePTFE) tubes. These tubes have a microporous structure which allows natural tissue ingrowth and cell endothelization once implanted in the vascular system. This contributes to long term healing and patency of the graft. Grafts formed of ePTFE have a fibrous state which is defined by the interspaced nodes interconnected by elongated fibrils.
One disadvantage of current thin-walled or thicker-walled implantable ePTFE tubes is their tendency to kink when subjected to bending forces or concentrated external radial forces. Kinking and luminal constriction can occur during or subsequent to implantation. Such kinking is normally undesirable and poses a risk to the patient.
Accordingly, in applications where kinking is likely, vascular grafts have an additional support structure to prevent kinking. Typically, external support structures, such as helical coils, are bonded around the outer surface of the ePTFE tube. Alternatively, individual rings may be bonded to the outer surface of the ePTFE by injection molding.
Such additional support structures have several disadvantages. For example, the additional support structures are normally bonded to the outer surface of the ePTFE tube thereby increasing the outer diameter of the graft in the regions of the support structures. As a result, implantation of the graft can become more difficult. For example, when tunneling through tissue is required to implant the graft, such as in vascular access applications, a larger cross-sectional tunnel area is required to allow for insertion of the graft.
Another disadvantage of grafts having added support structures is that they are often made from materials which are different from the material of the graft wall and require added processing steps such as heat bonding or additional materials such as adhesive to adhere the support structure to the graft. Differential shrinkage or expansion of the external support structure relative to the ePTFE tube can cause the bond to weaken and/or the graft to twist significantly. Separation of the support structure from the graft is obviously undesirable. Additionally, twisting will normally distort the printed linear guideline which typically runs the length of the ePTFE tube and is used by practitioners to determine proper graft disposition to prevent implantation in a twisted configuration. Such distortion may result in the normally longitudinally linear guideline becoming helical or some other non-linear shape prior to implantation of the vascular graft in the patient, thereby defeating the purpose of the guideline.
Other ePTFE grafts have included external polymeric ribs which provide radial support to the lumen, but increase the outer diameter and wall thickness of the graft.
Thus, there is a need for PTFE tubes which are kink resistant without added support structures such as coils or rings and which do not increase the tube outer diameter.